Currently reported lightning surges are about two times in amounts more than those occurred a decade ago. It becomes more and more important to prevent people and electric apparatuses from voltage surge. In this field, in addition to human life protection, more technical solutions have centralized on ensuring good and continuous operation of electric apparatuses. In accordance with lightning protection zone principle of VDE V 0185 Part 4, IEC 61643-11 stipulates specifications for sub-distribution network (Grade II) and terminals (Grade III), as well as gas discharge tubes at building entrances (Grade I).
The gas discharge tubes have been widely used for protection of electric apparatuses, sub-distribution networks and NPE (Neutral Protective Earth) at the building entrances. Since L-N (Phase-Neutral) protection at the building entrances requires high follow-current capacity of a product, the Grade I L-N protection usually uses, heretofore, conventional air sparks gaps with triggering devices, which are basically suitable for the use with high currents and voltages, but are cumbersome, expensive and voluminous.
Air gaps have mainly the following defects:    1. Voltage protection level is higher than 3000V.    2. The performance thereof is unstable during its life service life, and the open gap is susceptible to climatic environment and contamination.    3. Explosive flames or shock waves could occur during operation.    4. There is a requirement to keep installing distances thereof from nearby appliances or devices.    5. Additional trigger lines are required and arcs are extinguished by means of an auxiliary device.
In order to overcome the defect of insufficient open air gaps possible technical solution in relation to a closed gas discharge tube is to connect a plurality of gas discharge tubes in series so as to increase an arc voltage, thereby intercepting the follow current. If it is assumed that a single-layered arc voltage is only about 18V (in accordance with the gas discharge principle), 15 single-layered gas discharge tubes are necessary to be connected in series to achieve 270V arc voltage, which cannot meet market demands from the aspects of cost and volume.
To this end, some products use a single-layered gas discharge tube with multi-layered discharge gaps therein to increase the arc voltage, so as to reduce the cost and volume of the product. However, the defect of a lateral discharge between gaps cannot be avoided, and there is a limitation in enhancement of arc voltage and follow-current capacity, such that said products fail to be widely used.
There is a desire to provide a tube of a ceramic material which may be used as a gas discharge tube wherein the electric discharge occur in the center of the tube. Another desire is to provide a multi-layered gas discharge tube made of such a multi-step tube, which has the advantages of high arc voltage, high follow-current capacity, stable performance, small volume and easy installation.